Phase equilibria and critical behavior of squarehyphen;well fluids of variable width by Gibbs ensemble Monte Carlo simulation

摘要

The vaporndash;liquid phase equilibria of squarehyphen;well systems with hardhyphen;sphere diameters sgr;, wellhyphen;depths egr;, and ranges lgr;=1.25, 1.375, 1.5, 1.75, and 2 are determined by Monte Carlo simulation. The two bulk phases in coexistence are simulated simultaneously using the Gibbs ensemble technique. Vaporndash;liquid coexistence curves are obtained for a series of reduced temperatures between aboutTr=T/Tc=0.8 and 1, whereTcis the critical temperature. The radial pair distribution functionsg(r) of the two phases are calculated during the simulation, and the results extrapolated to give the appropriate contact valuesg(sgr;),g(lgr;sgr;minus;), andg(lgr;sgr;+). These are used to calculate the vaporhyphen;pressure curves of each system and to test for equality of pressure in the coexisting vapor and liquid phases. The critical points of the squarehyphen;well fluids are determined by analyzing the densityhyphen;temperature coexistence data using the first term of a Wegner expansion. The dependence of the reduced critical temperatureT@Bc=kTc/egr;, pressurePast;c=Pcsgr;3/egr;, number density rgr;@Bc=rgr;csgr;3, and compressibility factorZ=P/(rgr;kT), on the potential range lgr;, is established. These results are compared with existing data obtained from perturbation theories. The shapes of the coexistence curves and the approach to criticality are described in terms of an apparent critical exponent bgr;. The curves for the squarehyphen;well systems with lgr;=1.25, 1.375, 1.5, and 1.75 are very nearly cubic in shape corresponding to nearhyphen;universal values of bgr; (bgr;ape;0.325). This is not the case for the system with a longer potential range; when lgr;=2, the coexistence curve is closer to quadratic in shape with a nearhyphen;classical value of bgr; (bgr;ape;0.5). These results seem to confirm the view that the departure of bgr; from a meanhyphen;field or classical value for temperatures well below critical is unrelated to longhyphen;range, nearhyphen;critical fluctuations.